Object caching is a network technology used by web-based services in modern distributed systems for reducing latency, supporting disconnected operations, and improving user experience. This technology is becoming more popular with the increasing use of mobile devices such as smartphones and the development of cloud computing technology. Moreover, it is becoming a reality that a user of a web-based service often owns multiple terminals such as a desktop, a laptop, a tablet, a smartphone, etc., and the user can choose any of the terminals for accessing the service at any moment. This situation poses a significant challenge for object caching on how to keep objects cached at different terminals up-to-date. For example, if a user updates his or her calendar from a mobile phone, then the calendar cached at a desktop will be stale and must be updated accordingly. In those distributed systems involving hundreds of millions of users and trillions of objects, this is a non-trivial task.